Recently, the automobile exhaust emission regulation has become increasingly stringent, and in the context of such a circumstance, three-way catalysts are required to have further higher three-way purification functions, in order to meet the stringent exhaust emission regulation. This issue has hitherto been handled by supporting a large amount of noble metals on a carrier such as alumina in three-way catalysts. However, it is apparently a major cause of remarkable increase of a price of the three-way catalyst to support large amounts of noble metals. In addition, recently, prices of noble metals have been escalated worldwide; as a result, prices of three-way catalysts are increasing more and more, and accordingly it is urgently required to lower the prices of three-way catalysts.
Some catalysts whose contents of noble metals are decreased but whose three-way function is maintained as much as possible have already been proposed. For example, a three-way catalyst structure obtained by coating a substrate with a mixture of alumina and a rare earth oxide to form a layer of the mixture, and supporting rhodium and at least one noble metal selected from platinum and palladium on the layer of the mixture in an amount of about 0.7 to 5% by weight of the substrate is proposed (JP 2000-325792 A).
Also, a three-way catalyst structure obtained by mixing alumina powder with calcined particles of ceria/zirconia loaded with noble metals in predetermined contents, and supporting the mixture on a substrate, wherein each of the contents of platinum, palladium, and rhodium is from about 0.1 to 2% by weight, and the total content of the noble metals is from about 0.2 to 4% by weight is also proposed (JP 2001-232200 A).
Although the amounts of the noble metals contained in these catalysts are decreased compared with those in conventional three-way catalysts, it is required to further decrease the amounts of the noble metals.
Under such a circumstance, it is strongly required to develop a catalyst for catalytically removing carbon monoxide, hydrocarbons, and nitrogen oxides from combustion exhaust gas generated by combusting fuel near at the stoichiometric air to fuel ratio by bringing the exhaust gas into contact therewith, the catalyst having remarkably decreased amounts of noble metals compared with those in conventional three-way catalysts, being able to meet the stringent exhaust emission regulation, and having an excellent three-way purification function from a low temperature region.